The present invention relates to roller bearings and particularly to structures for effecting roller skew control.
The concept of controlling roller skew is not new per se. This concept and the theory involved are fully discussed in Kellstrom et al U.S. Pat. No. 3,990,753 for ROLLER BEARING COMPRISING ROLLERS WITH POSITIVE SKEW ANGLE issued Nov. 9. 1976 owned by the assignee of the present application and incorporated herein by reference (referred to herein as "Kellstrom Patent"). In the Kellstrom Patent, cause of roller skewing in a spherical roller bearing is shown to be due to residual pivotal moments arising in the contacts with the raceways. Roller skew of small magnitude in a direction defined as positive, was shown to have beneficial effects on the bearing friction, heat generation and ultimately on fatigue life. Kellstrom described various techniques for controlling the pivotal moments to achieve positive skew. These techniques all involved raceway changes. For example, one technique involved adjusting the friction coefficient through surface roughness control. Another involved changes in the raceway conformities and a third skew control technique involved provision of relief areas in one or both raceways. In all of these techniques the roller profile was conventional. Even though it has been found that the Kellstrom techniques are all feasible and may be effectively utilized to achieve the desired positive skew control, it may be difficult as a practical matter to control them all sufficiently accurately in manufacture to assure the desired slightly positive skew attitude of the loaded rollers in all bearings over a wide range of load, speed and lubrication conditions as explained in more detail below. The present invention is designed to provide an alternate and somewhat more versatile means of achieving positive skew control by selectively varying the roller profile in a predetermined manner while maintaining a constant radius of curvature for the inner and outer raceways.
In the Kellstrom raceway profiling or recessing technique, it has been found that if the geometric relationships between the rollers and the raceways change during operation of the bearing under load the pressure distribution is altered and the theoretically designed static condition to produce the desired small positive skew will in fact produce gross positive skewing of the rollers or even negative skew. Both of these conditions are obviously undesirable. The relative geometric changes can occur for example, in a double row self-aligning spherical roller bearing where the inner and outer rings move relative to one another to accommodate for operating conditions and thus change the force relationships. Further the Kellstrom technique of selectively varying the coefficients of friction of the raceways has limitations because of the presently relatively limited choice of bearing materials. Moreover controlling friction coefficients by surface roughness is a difficult manufacturing technique. Furthermore, surface roughness changes during the life of the bearing by reason of wear which of course changes the skew control. Also, friction is strongly influenced by the degree of elasto-hydrodynamic film, thereby impacting the effectiveness of surface roughness as a means of skew control.
In accordance with the present invention, the roller profile is selectively varied and the geometric relationships between the roller and the raceways remain constant even during adjusting movements of the raceways relative to one another in a self-aligning bearing. It has also been found that the principle of the present invention compensates for adjustments in the bearing to take up normal internal clearances and also geometry changes which occur when the bearing is subjected to varying load conditions. In summary, the present invention provides a skew control technique wherein the geometric relationships between the roller and the raceways remain the same either during misalignment or under load. Accordingly, the desired moment relationship to provide the small positive skew remains essentially unchanged.